Simulating rounded edges on a mesh
I recommend using a Normal Map to simulate the effect of a bevel. It doesn't add any mesh either (yes, even modifiers add mesh, except they are not directly applied to the object in Edit Mode). I wrote a tutorial on my blog for it.
http://renderedsmoothie.wordpress.com/2014/05/31/fake-bevels-using-normal-maps-blender-cycles/
(But here's the full post in a quote ;) )

First, we have a basic Cube. We need to UV-unwrap it. Press the U key, and click Smart UV Project to quickly unwrap this model. Next, duplicate your model and go to Edit Mode. Bevel that model by clicking Ctrl+B. (You can use the scroll wheel to adjust the quality of the bevel, and move your mouse to adjust the size of the bevel).

When you go back to Object Mode, you will notice that your cubes are overlapping. That’s great! Except that the beveled cube needs to be slightly larger.

Scale the beveled cube up just a small but so that you can not see the cube underneath.

Next, select your non-beveled cube and go to Edit Mode. Visit the UV/Image Editor window and create a new image. It can be whatever size you want. After you have your image created, go back to Object Mode on the cube, and follow these directions very carefully:

Select the Beveled Cube FIRST
(Go into Wireframe view mode for this one) Select the non-beveled cube SECOND.
Go into Edit Mode (Should be on the non-beveled cube)

You need to make sure you are in Blender Render Engine for this step. This is where we bake the Normals. In the Properties Window, find the Bake header. Change the Bake Type to Normals, set the Normal Type to Tangent, and check Selected to Active. Make sure you have your New Image in the Image Editor Window before baking. Then click Bake.

Shortly, you will see a Normal Map pop up on the Image Editor screen! Go ahead and Save the image so you don’t loose it from Blender’s Cache. Move the Beveled cube to another layer (by pressing the M-Key in the 3D Viewport) or Delete it. Next, switch back to Cycles Render. Cycles uses a special Node for Normal Maps. It’s called the Normal Map Node. Anyway, add that node to your node setup and plug it into the Normal plug of any shader. In the photo below, I have made a basic glossy and diffuse material setup using my “Bump-map-Normal-map”.